Typical WIM systems comprise two induction loops for the classification of vehicles as well as two series of WIM-sensors, which are built into a road spaced apart approx. 4 m from one another. The WIM-sensors detect signals on the basis of the loads that are transferred from the vehicle via the individual wheels in each case to the road or to the sensor. With the aid of ascertained calibration factors or calibration functions, the signals are finally converted into static or dynamic loads. It has been shown that the dynamic loads can diverge from the static loads, this being due in particular to the aerodynamics of the vehicles and the condition of their springs. In order to ascertain a weight of a vehicle, the individual ascertained loads of all the wheels assigned to this vehicle are finally added up. The same applies to vehicles with trailers. In order to increase the accuracy, a plurality of such WIM systems behind one another is sometimes provided and the ascertained measurement values as are evaluated.
WIM sensors are if possible built into straight sections of road, where they are passed over by vehicles experiencing as smooth a ride as possible. In particular, the vehicles should not be rocked by curves, gradients or slopes or by bumps, since this would falsify the measurement.
The measurement data pass into a WIM evaluating unit, which is typically housed in a cabinet at the roadside. From there, they are conveyed via lines and/or transmission antennas to a monitoring station, where the necessary measures are finally initiated by the persons responsible when, for example, permitted weights are exceeded. The measurement data in particular allow information to be derived about the load acting on the road, in order that surfacing work for example can be carried out in good time before considerable damage occurs.
A calibration of a weighbridge for rail vehicles is described in DE 10115490. For this purpose, a reference vehicle provided with vehicle weighing devices above each wheel is precisely weighed statically. The reference vehicle then passes over the weighbridge. The latter comprises a section of rail with integrated measurement sections, in which the load can be detected dynamically and transmitted to an evaluating unit. For the calibration, the weights ascertained statically on the reference vehicle are finally matched with the rail loads ascertained dynamically. Here too, dynamic loads are calibrated through static loads, as described at the outset. Errors caused by the aerodynamics of the vehicles and the condition of the springs also occur here.
The WIM sensors have to be recalibrated from time to time in order to permit the reliable determination of the weights of the vehicles passing over them. Nowadays, such a calibration is carried out with a statically weighed lorry. The latter passes over the measurement point to be calibrated between 10 and 70 times in order to achieve statistical reliability against random variations. These variations usually originate from the spring behavior of the vehicle.
This process is very time-consuming and expensive and is carried out about once per year for each WIM measurement point.